The invention relates to the transmission of data signals from a driver to one or several receivers on an electronic card used in a data processing device and relates in particular to a device enabling the reflection effects between a driver and the receivers of the card to be canceled.
Electronic cards are used in the devices of a data transmission network for transmitting high speed signals. Such cards can include a plurality of drivers each of them being connected by a net to one or several receivers. Accordingly, each net includes three parts: the driver characterized by its output impedance equivalent to a resistance, the transmission medium characterized by its characteristic impedance, its propagation delay and its topology (point to point, multipoint daisy chain or multipoint star) and the receiver characterized by its input impedance equivalent to a capacitance.
The problem to be solved with networks between the driver and the receivers is to obtain a high signal integrity at each receiver input level, and therefore to suppress as much as possible the reflections between the driver and each receiver. Unfortunately, the cards are designed in taking into account dimensional constraints imposing the floor planning of the components inside the card. Therefore, it is not possible to place the receivers anywhere on the card, and in particular, it is not possible to group them. Such constraints result in an optimization in the choice of the best topology to be used.
For critical high speed nets, only point to point topologies with a terminating resistance enable to solve the problem by introducing a resistance at the output of the driver such that the total output resistance is equal to the characteristic impedance of the net and to add a terminating resistance having a value equal to the characteristic impedance of the net.
But, in order to reduce the total number of components on the card, a single driver should be able to drive several receivers with the following requirements, fastest propagation delay through the transmission medium, fastest transition time across the threshold region of the receiver, no glitch allowed in the threshold region of the receiver and limited overshoot/undershoot outside the threshold region. To meet these requirements, the terminated multipoint star is to be avoided insofar as a resistance terminating each receiver would result in a very important energy consumption for the driver device.
The best way would be to use a terminated multipoint daisy chain. But such a solution consisting in using a terminating resistance the value of which is equal to the characteristic impedance of the net, results always in an important energy consumption and there are remaining reflections with the high speed signals caused by the parasitic input capacitance of the receiver.
Accordingly, the object of the invention is to provide a device for cancelling the effects of the reflections in a multipoint topology wherein a driver transmits high speed data signals to a plurality of receivers.
The invention relates therefore to a device for cancelling the effects of the reflection in a signal transmission system including a driver and a plurality of n receivers wherein signals are sent according to a multipoint topology from the driver to the receivers, each receiver having an internal capacitance and sending back reflection signals to the driver each time a signal is transmitted thereto by the driver. The device comprises circuit means causing the signal resulting from the sequential reflections due to a given receiver and then due to the driver to have the same magnitude but the reverse sign as the sum of all signals received in the given receiver resulting from the reflections due to all receivers, and the net linking the driver to each receiver comprises delay means enabling the propagating time of a signal sent from the driver to this receiver to be identical for each receiver, whereby the total sum of all reflection signals arriving in the given receiver at the same time is equal to zero.